Genetic analysis has shown that a newly isolated dominant, mutant allele (wDZL) at the white locus of Drosophila melanogaster is highly mutable and produces sequence rearrangement mutations of all varieties predicted by a transposable-element hypothesis for the physical basis of this mutable allele. We have recently cloned DNA sequences from the white locus region. In situ hybridization analysis demonstrates that the cloned sequences reside very near and probably within the region containing the white locus genetic elements. These same data corroborate the results of cytogenetic analysis mentioned above and thus support the transposable-element hypothesis.